Define and use the terms focal length, principal axis and principal focus (focal point)

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Cambridge IGCSE Physics - Thin Lenses

Thin Lenses

This section explores the properties of thin lenses, focusing on key terms and their application in understanding how lenses form images.

Key Terms

Focal Length

The focal length (f) of a lens is the distance between the lens and its principal focus. It is a fundamental property of a lens and determines its ability to converge or diverge light.

  • A convex lens has a positive focal length.
  • A concave lens has a negative focal length.

Principal Axis

The principal axis is an imaginary straight line that passes through the center of the lens. It is perpendicular to the surface of the lens.

Suggested diagram: A convex lens with the principal axis shown as a horizontal line passing through its center.

Principal Focus (Focal Point)

The principal focus (or focal point) is the point where parallel rays of light converge (for a convex lens) or appear to diverge from (for a concave lens) after passing through the lens.

The distance from the lens to the principal focus is the focal length.

Suggested diagram: A convex lens with parallel rays of light converging at the principal focus.

Lens Formula

The lens formula relates the focal length of a lens to the object distance (u) and the image distance (v) of an image formed by the lens.

The formula is:

$$ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} $$

Where:

  • f = focal length of the lens
  • u = object distance (the distance from the object to the lens)
  • v = image distance (the distance from the image to the lens)

Object and Image Distances

It's important to define object and image distances:

  • Object distance (u): The distance of the object from the lens. It is considered positive if the object is on the same side of the lens as the incoming light and negative if the object is on the opposite side.
  • Image distance (v): The distance of the image from the lens. It is positive if the image is on the same side of the lens as the incoming light and negative if the image is on the opposite side.

Summary Table

Term Definition Type of Lens Sign
Focal Length (f) Distance between the lens and the principal focus. Convex or Concave Positive (convex), Negative (concave)
Principal Axis Imaginary straight line passing through the center of the lens, perpendicular to its surface. N/A N/A
Principal Focus Point where parallel rays of light converge (convex) or appear to diverge from (concave). N/A N/A
Object Distance (u) Distance of the object from the lens. N/A Positive or Negative
Image Distance (v) Distance of the image from the lens. N/A Positive or Negative